This invention relates to a device for microwave heating.
At microwave heating of material with relatively low microwave losses, i.e. low effect absorption, the microwave applicator in most cases must be designed with an unpractically great length.
It is difficult, moreover, at the heating of oblong material with low microwave losses to achieve a uniform effect absorption.
The present invention eliminates the aforesaid shortcomings.
The power P.sub.T transported along an applicator decreases according to e.sup.-2.alpha.x of the function, where .alpha. is a constant depending on the microwave losses of the material and the geometry of the applicator, and x is the length coordinate of the applicator.
The power absorbed per length unit in the material can be written as EQU P.sub.f =.differential.P.sub.T /.differential.x=2.alpha.e.sup.-2.alpha.x
where .alpha. is a relatively small number at materials with low microwave losses.
As an example can be mentioned, that a material with a low dielectricity constant .epsilon.=2 and with the loss angle tan .delta.=0.001 which is heated in a normal waveguide with a width=60 mm at a frequency=2450 MHz, after 10 m still has absorbed only about 65% of the power supplied.
The transported power P.sub.T can be expressed as stored energy (W) per length unit (1) times propagation velocity (V.sub.g) EQU P.sub.T =W/1.multidot.V.sub.g
At constant transported power, thus, the stored energy W per length unit increases when the propagation velocity V.sub.g decreases.
The aforesaid can be read, for example, from Collin: "Field Theory of Guided Waves", chap. 9.6.
By holding V.sub.g sufficiently small, it is thus possible to increase .alpha. to a value acceptable for obtaining a reasonable applicator length.
A waveguide, however, proceeds to cut-off when V.sub.g proceeds to zero, and is near cut-off when V.sub.g is small. Therefore the risk is great that supplied power is reflected totally already before it has arrived at the material to be heated.